I love GTA1 simply for what it was. It was amazin at the time and I had loads of fun with it.

But also I hold GTA3 in high regard simply because it was 3d and amazing all over again, it felt gritty and maybe because I was a bit younger it felt a bit more 'scarier', well not scary but just gritty and believable. VC and SA while being good, kinda felt less serious. So those two i'd say.